It is well-documented that disorders of skeletal tissues and mineral metabolism cause numerous significant health problems on world-wide basis.
In humans, the maximum bone mass occurs between the age of 15 and 40 and is referred to as “peak bone mass.” After such peak bone mass age, bone mass begins declining gradually and the mechanical strength of the bone is accordingly reduced. Consequently, when mechanical strength declines to a certain level, the individual is at greater risk of bone fracture. This natural occurrence is called osteoporosis if severe enough to be pathogenic.
The speed at which bone loss occurs differs among individuals, and especially with respect to gender. In females, the speed of bone loss accelerates immediately after menopause (See FIG. 1) because of a significant decline in available estrogen, a hormone which plays a critical role in maintaining healthy bone metabolism. Postmenopausal osteoporosis constitutes an important clinical problem because it afflicts significant numbers of women. Notably, the ratio of female to male osteoporosis patients is 3:1.
The majority of bone diseases are characterized by loss of bone minerals, weakening of bones and consequently, an increase of the frequency and severity of bone fractures, which are called “pathological fracture.” In the elderly population, this has significant social ramifications as well, as many of those with bone fractures have difficulty with mobility, which often leads to the deterioration of other mental and physical functions, resulting in dementia, muscular weakness and/or fatigue. In addition, morbidity and pain are significantly increased by thrombotic events, such as pulmonary embolism which can occur as a result of hip or pelvic fractures.
In the United States alone, it is said that 52 million women over age of 45 will suffer from osteoporosis by 2000. Current worldwide osteoporosis population is around 200 million. Annual incidence of pathological fracture in the United States alone is approximately 1.5 million. It is estimated that annual medical costs for those osteoporosis patients in the United States and world are $14 billion and $60 billion, respectively.
Renal failure is also a significant health problem related to mineral metabolism and skeletal formation, and the number of its patients is increasing rapidly. Renal function is declining gradually over several to ten years period in these patients. When the renal function becomes approximately a quarter (¼) of the healthy level, the patients are classified to chronic renal failure. When it becomes approximately one sixth (⅙) thereof, they need to start dialysis and are called end stage renal disease (ESRD). In patients with chronic renal failure, serum levels of important minerals such as calcium and phosphate lose their normal homeostasis, which results in malformation of skeleton. It is called renal osteodystrophy (ROD), which is a secondary osteoporosis from renal failure. ROD can also cause pathological fracture like osteoporosis. The prevalence of end stage renal disease (ESRD) in the United States is rapidly increasing and about to reach 300 thousand in 2000. ROD affects most ESRD patients.
There are several other diseases of skeletal tissues and mineral metabolism such as Paget's Disease, rickets, osteopetrosis, hyperparathyroidism, and so forth and a number of patients are affected by these diseases.
Metabolically, bone is a highly active organ with bone resorption and formation occurring continuously (remodeling). Bone resorption is facilitated by osteoclasts which are differentiated from monocyte/macrophage lineage cells. Osteoclasts adhere to the surface of bone and degrade bone tissue by secreting acids and enzymes. Osteoblasts facilitate bone formation by adhering to degraded bone tissue and secreting bone matrix proteins, which are mineralized mostly by calcium and phosphate. Osteoblasts differentiate into bone cells (osteocytes), and become a part of bone tissue.
Numerous experimental approaches have been attempted to either accelerate bone formation or diminish bone resorption. For example, growth factors such as BMPs (bone morphogenetic proteins), TGFβ (transforming growth factor β), IGF (insulin-like growth factor), and fibroblast growth factor (FGF) are known to have potent biological activities in bone formation. In particular, a few subfamily molecules of BMP such as BMP-2 is regarded as one of the most potent growth factors for hard tissue. However, these factors have not been developed as therapeutic agents for systemic bone diseases. It is because none of them can be delivered to the bone selectively and some of these factors such as BMPs convert soft tissue into hard tissue. It is called ectopic calcification and is a critical adverse effect for them when they are used systemically. Further, the processes of bone formation and resorption are so closely connected and that makes selective increase of bone formation or selective inhibition of bone resorption extremely difficult.
Currently, there is a need for an effective treatment for bone loss. Therapeutic agents such as estrogen, calcitonin, vitamin D, fluoride, Ipriflavon, bisphosphonates, and a few others have failed to provide a satisfactory means of treatment. (Gennari et al., Drug Saf. (1994) 11(3):179–95).
Estrogen and its analogues are frequently administered to patients with postmenopausal osteoporosis. Estrogen replacement therapy involves administration of estrogen just prior to or after the onset of menopause. However, as is often the case with steroid hormones, the long term use of estrogen has significant adverse effects such as breast and other gynecological cancers (Schneider et al., Int. J. Fertil. Menopausal Study (1995) 40(1):40–53).
Calcitonin, an endogenous hormone produced by the thyroid, binds selectively to osteoclasts, via its receptor, and inactivates them. Since the osteoclast is the only cell which can dissolve bone tissue, calcitonin binding can block or slow down bone degradation caused by the osteoclast. However, this biological mechanism is very short-lived, as the osteoclasts become tolerant to this drug relatively quickly. Therefore, the use of calcitonin does not provide an effective therapeutic option.
Fluoride has been shown to increase bone mass when it is administered to humans. However, while bone mass is increased, mechanical strength is not. Therefore, despite the increase in apparent bone mass, the risk of fracture remains (Fratzl et al., J. Bone Mineral Res. (1994) 9(10):1541–1549). In addition, fluoride administration has significant health risks.
Ipriflavon has been used to treat osteoporosis in limited areas in the world. However, the actual efficacy of this compound is questionable and it is not widely accepted as a useful therapeutic agent for bone diseases.
Bisphosphonates are compounds derivatized from pyrophosphate. Synthesis involves replacing an oxygen atom situated between two phosphorus atoms with carbon and modifying the carbon with various substituents. While bisphosphonates are known to suppress bone resorption, they have little effect on bone formation. Furthermore, bisphosphonates adhere to the bone surface and remain there for very long time causing a long-term decrease in bone tissue turnover. As bone tissue needs to be turned over continuously, this decrease in turnover ultimately results in bone deterioration (Lufkin et al., Osteoporos. Int. (1994) 4(6):320–322; Chapparel et al., J. Bone Miner. Res. (1995) 10(1):112–118).
Another significant problem with the agents described above is that with the exception of fluoride and ipriflavon, they are unsuitable for oral administration, and thus, must be given parenterally. Since bone disorders are often chronic and require long-term therapy, it is desirable that therapeutic agents be suitable for oral administration.
In summary, a significant need exists for a therapeutic agent which can prevent or treat bone loss. In particular, a new drug that can selectively increase bone formation and/or number of osteoblast without affecting bone resorption or soft tissue is highly desired.
Another major health problem relating to skeleton and mineral metabolism is that with teeth. In the United States alone, it is estimated that 67 million people are affected by periodontal disease and that the annual cost of its treatment is approximately $6.0 billion in 2000. It is said 90% of the entire population experience dental caries in their lives. The annual cost to treat them is over $50 billion per year in the United States alone.
Dental caries are a universal disease and affects children and adults. Periodontal disease, on the other hand, affects mostly adults, and in particular, the aged. In many cases, the patient's gum is inflamed and destroyed, and the alveolar bone that supports the teeth is deteriorated. Cement that composes the core of the root is also damaged, and subsequently, teeth fall out. One of the most common treatments for tooth loss involves the use of a dental implant. An artificial implant (osseointegrated dental implants) is placed in the space where the tooth was lost. In severe cases, an entire denture is replaced by implants. However, implants frequently loosen, or fall out because their fixation on the alveolar bone is not always successful. Since alveolar bone is somehow damaged in these patients, the implant cannot always be supported well by alveolar bone. When alveolar bone is severely damaged, autogenous bone grafting is performed. In this case, a bone graft taken from another skeletal tissue of the same patient is grafted in the damaged alveolar area so that the hard tissue is regenerated and sinus is elevated there. Since these treatments require expensive bio-compatible materials and/or highly skilled techniques, the cost of treatment is usually very high.
It is believed that dental caries are caused by acidic condition in the oral cavity. For instance, sugars are converted to acid and dissolve the surface of the teeth. Although only enamel and a part of dentin is affected in many cases, the damage can reach the pulp cavity in severe cases that cause significant pain. The most typical treatment is filling the caries lesion with non-degradable materials such as metals or metal oxide. Treatment of dental caries mostly depends upon those materials and the techniques by the dentists, which is often expensive.
Although a few therapeutic agents have been developed and used in dental area, they are generally only anti-inflammatory drugs, analgesics, and antibiotics. No generally effective therapeutic agent that directly improves periodontal hard tissues has been developed.
Another major clinical problem in mineral metabolism is excessive loss or waste of phosphate (PO4) out of the body system. Phosphate plays variety of important roles in all living creatures. In vertebrates, phosphate is a major component of their skeleton. In all animals, phosphate is an essential component to build polynucleotide chains and cell membranes; phosphorylation and dephosphorylation of sugars and nucleotides are the most essential reactions in energy generation and consumption; and phosphorylation and dephosphorylation of proteins, sugars, and lipids are indispensable reactions for signal transduction in the cells. Therefore, a shortage of phosphate could even result in death.
In mammals, phosphate concentration in the body fluid is controlled within a range that allows all normal biological functions in the body. The kidney is the most important organ for controlling phosphate levels in the body. Glomeruli in the kidney filter phosphate constantly to the urine, and proximal tubules usually reabsorb approximately 80% of this filtered phosphate. If this reabsorbing function is damaged, excessive phosphate is lost into the urine, resulting in various clinical problems.
For instance, it is well known that the majority of kidney transplant patients experience excessive renal phosphate leakage, because the transplanted kidneys only marginally reabsorb the urinary phosphate to the circulation. The reasons for this poor reabsorbing activity on the part of transplanted kidneys are unknown. It frequently causes the patients malnutrition and secondary osteoporosis. This problem cannot be treated by a simple exogenous supplementation of phosphate. Similar renal phosphate leakage with unknown pathology is often observed in pediatric medicine, with outcomes such as malnutrition or growth retardation.
Health problems associated with circulating phosphate shortage is not limited to humans. Milking cows sometimes suffer from hypophosphatemia (too low phosphate in the blood) by overproduction of the milk. It not only deteriorates the nutritional quality of the milk but also often make the cows useless for milk production. It is as relatively common problem in dairy farms.
Clearly, there is a significant demand for a therapeutic agent that promotes regeneration of alveolar bone and/or teeth, increases the number and activity of odontoblasts/osteoblasts that help form dental tissues, and reduces renal phosphate secretion.